Method for rapid activation or preconditioning of porous gas purification substrates

ABSTRACT

A method is described for rapid and economical activation and/or preconditioning of gas purification substrates by providing forced convection of the preconditioning or activating gas through the pores of the substrate. The gas is pumped into the substrate-containing vessel and raised to an elevated pressure, which is maintained for a short predetermined time, followed by venting of contents of the vessel. The vessel is again pressurized with the purging gas to an elevated level, and the elevated pressure is maintained for a short predetermined time, followed by venting of the vessel. This cycle is repeated as often as needed or desired. Activation and/or preconditioning can be accomplished in a much shorter time and with much less gas usage compared to diffusion preconditioning and activation processes. This process is particularly suited for preconditioning and activation of gas purifier substrates for decontamination of gases down to ≦1 ppm contaminants.

CROSS-REFERENCES TO RELATED APPLICATIONS

[0001] This application is a continuation of U.S. patent applicationSer. No. 10/173,335 filed Jun. 14, 2002, which is incorporated herein byreference in its entirety.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The invention relates to the activation or preconditioning, orboth, of porous gas purification substrates. More particularly itrelates to activation or preconditioning of such substrates rapidly andwith reduced gas usage.

[0004] 2. Background Information

[0005] Purification (decontamination) of many types of gases is donebypassing a contaminated gas or gas mixture over a porous substratewithin a vessel. The contaminants in the gas are trapped on active siteson the surface of the substrate and the gas upon exiting from the vesselhas a much reduced concentration of contaminants. This type of gaspurification is commonly used to reduce the contaminant concentration ofa manufacturing process gas which is subsequently to be used in themanufacture of high purity materials such as semiconductor wafermaterials and prosthetic devices. In such purification processes thecontaminate level of the gas is often reduced to or below 1 ppm and inmany processes can be reduced into the range of parts per billion (ppb)and even in some cases into the parts per trillion (ppt) range.

[0006] Substrates may be in the form of flat or curved plates, smallshaped objects such as rings, spheres, saddles or the like, or beparticulate or granular materials. While the actual substrate form isoften a matter of choice based on factors such as acceptable gaspressure drop through the vessel, the substrate normally must be highlyporous so that it has a high surface area, since contaminate removalfrom gases is essentially a surface phenomenon. Many porous substrateshave surface areas on the order of 100 square meters per gram (m²/g) orhigher.

[0007] When a new substrate material is initially placed into a gaspurification vessel the pores of the substrate are filled with a packinggas, which is simply environmental gas to which the substrate haspreviously been exposed. Commonly this is air or an inert gas. Since thepresence of the packing gas in the pores blocks access of thecontaminated gas to many of the active removal sites on the surface ofthe substrate, the packing gas must be removed by purging and thesubstrate saturated, usually with the same gas as will be purified or acomponent thereof, before the purification operation can begin. The sametype of initial gas removal must also be performed when a vessel hasbeen shut down and vented (such as for repairs) and is to be againplaced in operation. This removal and replacement process is commonlyreferred to as “preconditioning” of the substrate.

[0008] In some preconditioning processes chemical reactions may alsooccur, generating water vapor or other gaseous by-products. The flow ofthe preconditioning gas must also continue until the reactions haveceased and the gaseous by-products have been purged from the system.

[0009] There is an equivalent processes used when the active sites on asubstrate are of only limited decontamination activity initially. Suchsites must be “activated” by contacting them with an activating gas,causing them to become much more active for decontamination. Themechanism of activation is not important for this invention. What isimportant, however, is that the activating gas must come into contactwith the surface sites of the substrate in order to activate them. Thusthe purging gas must be forced to as many of the activation sites aspossible during activation. A particular substrate may require bothactivation and preconditioning, which may occur simultaneously or insequence, and may be accomplished either by different gases or by thesame gas.

[0010] It will be evident that for both preconditioning or activationprocesses it is important that the packing gas be removed from all areasof the surface of the substrate and that all sites must be contacted ifthey are to activated. While this is readily accomplished for thosesurface sites and areas to which easy access of a flowingpreconditioning or activation gas can be obtained, such as the outersurface of the substrate plate, object or granule, it becomes much moredifficult for those areas of the substrate that are deep within thepores of the substrate.

[0011] In past gas purification processes, activation andpreconditioning gases have been flowed through the vessel and across thesubstrate and have reached into the pores of the substrate by masstransfer/molecular diffusion. Very long activation or preconditioningperiods have been required since such diffusion occurs slowly,particularly as the gas traverses into greater depths of the pores. Itis quite common for it to require 24 to 48 hours for satisfactoryactivation or preconditioning of an entire substrate to be accomplishedby flow-generated mass transfer/molecular diffusion. In addition, suchdiffusion does not provide thorough activation or preconditioning, sinceas a pore narrows over its length, there is greater resistance todiffusion of the purging or activating gas through it, such that manysites requiring activation or areas requiring purging of packing gasessimply cannot be reached by the slowly diffusing gas within a reasonableperiod of time. During prolonged preconditioning or activation periodsrequired, it is not uncommon to have excessive exotherms occur withinthe substrate. In order to avoid such exotherms (which could damage thesubstrates) it is often necessary to limit the flow rate of the purginggas through the vessel, thus also reducing the rate of diffusion of thepurging gas into the pores and prolonging the activation orpreconditioning time period.

[0012] Forced convection purging of equipment has been used in some ofthe chemical and petroleum industries, but it has been with respect tomacro-scale processes in which only relatively coarse and limitedremoval of packing gases or limited activation of active sites has beenrequired. Such has not previously been known in or believed applicableto gas purification reactors and vessels in which ultra-high purity (≦1ppm contamination) must be accomplished.

SUMMARY OF THE INVENTION

[0013] The present invention overcomes the problems of activation andpreconditioning in prior art gas purification systems, and materiallyspeeds up activation and preconditioning, generally eliminates thelikelihood of excessive exotherms, and permits much more thoroughsaturation of the substrate with the activation and preconditioninggases. This is accomplished in the present invention by providing forcedconvection of the preconditioning or activating gases through the poresof the substrate. (For brevity herein, preconditioning and/or activationgases will sometimes be referred collectively or singly as “purging”gases.) In the process of the present invention a purging gas or gasmixture to be used for preconditioning or activating is pumped into thesubstrate-containing vessel, on which all outlet ports have been closed.The amount of gas used is sufficient to raise the gas pressure toseveral times the “atmospheric” pressure. The elevated pressure ismaintained for a short predetermined time and then the outlet ports ofthe vessel are opened and the contents of the vessel are vented toatmospheric pressure. This will include both purging gas and packing gasdisplaced during the pressurization. Promptly thereafter the outletports are closed and the vessel is again pressurized with the purginggas to an elevated level, and the elevated pressure is maintained for ashort predetermined time, again followed by venting of the vessel. Thiscycle is repeated as often as needed or desired.

[0014] We have determined that such pressurizing and venting cycles whenrepeated for at least two, preferably at least four, and more preferablyat least ten, times will result in most if not all of the sites beingactivated and most if not all of packing gas (and any gaseous byproductif a chemical reaction has occurred) being purged from the substrate andvessel. The forced convection of this process causes the purging gas tobe forced through essentially all of the narrowest portions of theporous substrate, such that virtually all activation sites andgas-containing recesses are reached by the purging gas, in a manner muchmore rapid and much more thorough then what is accomplished by diffusionof a purging diffusion. This results in not only much more rapid andcomplete activation or preconditioning, but also in the use of far lesspurging gas than is required for the long flow periods necessary fordiffusion activation or preconditioning.

BRIEF DESCRIPTION OF THE DRAWINGS

[0015]FIG. 1 is a graph from a typical prior art preconditioning processfor an ammonia purification vessel using gas-flow-generated masstransfer/molecular diffusion, showing the comparison of quantity ofpurging gas used versus temperature reached.

[0016]FIG. 2 is a graph from a preconditioning process for an ammoniapurification vessel using the forced convention of the presentinvention, also showing the comparison of quantity of purging gas usedversus temperature reached.

[0017]FIG. 3 is a composite of FIGS. 1 and 2 showing the directcomparison of the forced convection data (left edge of the graph;triangles) of this invention with data from the prior art diffusionprocess (extending all the way across the graph; closed circles).

DETAILED DESCRIPTION AND PREFERRED EMBODIMENTS

[0018] The present invention comprises the use of forced convection of apurging gas to activate sites throughout the pores and surface of asubstrate and/or to purge a packing gas from a substrate, all in a smallfraction of the time previously required for preconditioning oractivation by diffusion and with use (and resulting waste by venting) ofonly a small fraction of the purging gas previously needed by the priorart diffusion processes.

[0019] In the present process the purging gas (which may be theactivation gas, the preconditioning gas or a gas which serves bothpurposes) is pumped into the substrate-containing vessel, raised to aelevated pressure and maintained at that pressure for a shortpredetermined time, following which the contents of the vessel arevented to the atmosphere or to an “atmospheric” pressure collectionvessel. Promptly thereafter more purging gas is pumped into thesubstrate-containing vessel and raised to elevated pressure, maintainedat elevated pressure for a short determined time, followed by venting ofthe vessel contents to the atmosphere or an atmospheric pressure vessel.This cycle is repeated for as many times as necessary to reach thedesired level of activation of the active sites of the substrate and/orfor removal of all substantially packing gas within the substrate. Ifduring the preconditioning a chemical reaction also occurs whichgenerates moisture and/or another gaseous byproduct, the cycles mustalso continue until the chemical reaction has reached completion and allgenerated byproduct is also purged from the system.

[0020] We have found that the pressurize-and-vent cycle is convenientlyrepeated at least two, and preferably at least four, and more preferablyat least ten, times. There is no absolute maximum number of cycles, butin practice 200 cycles are anticipated to be sufficient for activationor preconditioning of almost all substrates, and in many casessignificantly less cycles (such as 10-100) will be quite adequate. Thepressurization is preferably raised to and maintained at a level of atleast two times the “atmospheric” pressure, and preferably at least fivetimes the atmospheric pressure. Normally each cycle will return to thesame elevated pressure level, but that is not required. By “atmospheric”pressure is meant the pressure of the environment into which the gas inthe vessel is vented following the pressurization portion of a cycle,which may conveniently be the open ambient environment or a capturevessel. Preferably however, one will vent to a subatmosphericenvironment, particular one with a strong vacuum, which may be as low as10⁻⁷ torr (1.33×10⁻⁵ Pa). The important criterion is that the pressuredifferential between the elevated pressure during pressurization and thepressure upon venting should be at least two times, and preferably atleast five times, the vented pressure. There is no absolute maximumdifferential, and it is contemplated that differentials as high as 10¹⁰times are feasible. Typically with vacuum venting differentials of 10⁸are convenient, which with atmospheric venting the differentials aremore usually on the order of 10⁴. The object is to have sufficientlyhigh pressure during the elevated pressurization period to force thepurging gas into and through essentially all parts of the substrateincluding the narrowest portions of the pores and into any smallcul-de-sacs within the pores, and then upon venting to have asufficiently high pressure differential so that most of the contents ofthe vessel will be evacuated quickly and thoroughly during the venting.The vessel contents being evacuated will contain not only a substantialamount of the purging gas but also a substantial amount of any packingor other gas which the purging gas will have displaced during thepressurization phase of the cycle.

[0021] Each cycle is relatively short. The amount of hold time at theelevated pressure will generally be in the range of ten seconds to tenminutes. Additional hold time is not usually advantageous, since theforced convection mechanism of the present invention works mostefficiently through multiple repeated cycles than by having extendedtimes within each single cycle. Having relatively short cycles alsosignificantly limits any occasion for an excessive exothermic reactionduring any individual cycle. There will normally be a small exothermthat occurs during the first few cycles, as will be seen in FIGS. 2 and3, but that exotherm normally dissipates quickly as most of the packinggas becomes removed and most of the sites become preconditioned oractivated during the early part of the process.

[0022] The present invention is useful to prepare substrates for use ina wide variety of gas purification processes, including those forpurifying both bulk gases and specialty gases. Among the bulk gaseswhich can be purified in a processes for which the present inventionprovide initial activation and/or preconditioning are hydrogen, oxygen,nitrogen, argon, hydrogen chloride, ammonia, air, carbon dioxide andhelium. Specialty gases included silane, germane, diborane, phosphineand arsine. All of these gases may also be in mixtures with either otheror with other gases, such as mixtures (blends) of the speciality gaseswith hydrogen, nitrogen or argon as the carrier gas, especially in whichthe dopant (non-carrier) gas concentration is from 50 ppm up to fivepercent of the mixture. It is preferred that the gas or gas mixture tobe decontaminated will be the same as the gas or gas mixture to be usedfor purging to accomplish preconditioning or activation, but the presentinvention also contemplates that a non-identical gas could be used inthe purging if its continued presence after purging or activation willnot adversely affect the purification of the contaminated gas. Thus forinstance, where the gas to be decontaminated is a mixture with a smallconcentration of the dopant gas, it might be desired to preconditionwith the principal component of the mixture (i.e., the carrier gas inthis case) alone as long as the substrate does not thereafter act toreduce the concentration of the dopant gas in the mixture duringdecontamination.

[0023] The present invention finds its most significant application inthe preconditioning and/or activation of substrates used in gaspurification processes and equipment in which the treated gas or gasmixture is to be decontaminated down to a level of no greater than 1 ppmof contaminants, preferably down to a level on the order of 1-10 ppb ofcontaminants, and more preferably down to a level on the order of about1-100 ppt.

[0024] The superiority of this process is illustrated in FIGS. 1, 2 and3, which illustrate preconditioning with ammonia to purge an ammoniadecontamination substrate of packing gas (nitrogen). It is conventionalto determine completion of preconditioning in a practical sense bymonitoring the temperature of the interior of the vessel. An exothermoccurs early in the preconditioning process as the packing gas isdisplaced. As the concentration of the packing gas decreases theexotherm dies away and the interior of the vessel reaches an equilibriumtemperature (which in the case of ammonia is about 20° C. [68° F.]),indicating that little or no significant amount of the packing gas isstill present and being purged. When the equilibrium temperature hasbeen reached and maintained for a period sufficient to confirm itspresence to the operator, the preconditioning process is deemedcomplete. The flow of contaminated gas can then be started and thedecontamination process will commence.

[0025] In FIG. 1 the substrate is shown as being preconditioned withammonia by prior art continuous gas flow through the vessel to producemass transfer/molecular diffusion of the ammonia through the pores ofthe substrate. It will be seen that almost 1200 liters of ammonia perliter of substrate must be flowed through the vessel before the exothermreaches its equilibrium temperature level, and another 200-400 litersmust be used before the presence of the equilibrium temperature isconfirmed sufficiently to warrant halting the preconditioning process.The overall time involved in the process shown in FIG. 1 was 9.5 hoursto initially reaching the equilibrium temperature and 2.5 hours toreaching a point at which the operator could reasonably conclude thatequilibrium temperature had in fact been established.

[0026] In the present invention, however, as illustrated in FIG. 2, thesystem is cycled through 10-11 cycles (each data point) before theequilibrium temperature level is reached, and only about 5 or so morebefore that level is confirmed, with the total use of only 60-80 litersof ammonia per liter of substrate, a 20-fold improvement over the priorart diffusion system of FIG. 1. Also the exotherm reached (43°-45° C.[110°-113° F.]) is no greater than is reached by the prior art diffusionprocess preconditioning. Of equal significance with respect to thesuperiority of this invention is that there was a five-fold decrease inthe amount of times needed to reach the initial equilibrium temperatureand confirmation point, as compared to the times needed for the priorart diffusion preconditioning process of FIG. 1.

[0027] The two graphs of FIGS. 1 and 2 are shown on the same grid inFIG. 3. The dramatic reduction in ammonia usage (and also inpreconditioning time) is evident in this Figure. It will be seen thatthe diffusion purging process has used more ammonia (and used more time)just for its first stage—reaching the peak of its exotherm—than did thepresent invention's forced convection purging for completion of itsentire preconditioning, including the period needed to confirm that theequilibrium temperature had been reached. Thus the process of thisinvention can accomplish preconditioning in a small fraction of the timeand with a small fraction of the gas usage as are required in the priorart diffusion preconditioning processes.

[0028] Not directly shown in the Figures but evident from them is theimportant improvement in costs of the present invention. It will berecognized that gas used for preconditioning cannot be recovered for useas a decontaminated manufacturing process gas, since upon exiting thevessel it will be contaminated with the packing gas or other materialsfrom within the vessel which it has displaced within the substrate. Notuntil the preconditioning process is complete can usable manufacturingprocess gas be obtained from the gas purifier. Since as noted above onenormally uses the same gas (or gas mixture) to precondition as will beused in the subsequent purification operation, the amount of gas usedduring preconditioning represents direct economic loss to the systemoperator. Thus in the examples shown in the Figures, the operator of thediffusion preconditioning process has lost some 1200 or more liters ofammonia while the operator of the present forced conventionpreconditioning process has lost only 60-80 liters. Even with a commongas such as ammonia, the economic value disparity is significant, and itwill of course be much greater when the gases used are expensivemixtures or speciality gases.

[0029] The nature of the gas decontamination vessel is not critical, noris the nature of the substrate. Each will be determined by the physicaland chemical properties of the gas to be purified, and since in thepreferred mode of this invention that will also be the gas used as theactivating and/or preconditioning gas, there will not be any problem ofincompatibility or of adverse effects with the forced convectionpreconditioning and/or activation of the present invention. Numerousdifferent gas decontamination vessels and substrates for a wide varietyof gases and gas mixtures are available commercially, including thoseavailable from the assignee of the present invention and patentapplication, Aeronex, Inc. of San Diego, Calif.

[0030] It will be evident that there are numerous embodiments of thepresent invention which are not expressly described above but which areclearly within the scope and spirit of the present invention. Therefore,the above description is intended to be exemplary only, and the actualscope of the invention is to be determined from the appended claims.

We claim:
 1. A method for preconditioning, activating or both of a gasdecontamination substrate within a vessel, which comprises: a. fillingsaid vessel containing said substrate with a purging gas and rasing saidpurging gas to an elevated pressure within said vessel; b. maintainingsaid purging gas at said elevated pressure for a predetermined period oftime; c. venting contents of said vessel to an environment having a gaspressure substantially less than said elevated pressure, such that muchof said purging gas and any contents of said vessel displaced by saidpurging gas are evacuated from said vessel; and d. repeating steps a.,b. and c. at least once, whereby said substrate within said vesselbecomes preconditioned or activated or both for subsequentdecontamination of a contaminated gas.
 2. A method as in claim 1 whereinsaid purging gas is the same type of gas as a gas which is to besubsequently decontaminated or is the same type of gas as a principalcomponent of a gas mixture which is to be subsequently decontaminated.3. A method as in claim 1 wherein the pressure differential between saidelevated pressure and said lesser gas pressure of said environment is atleast a factor of two.
 4. A method as in claim 3 wherein the pressuredifferential between said elevated pressure and said lesser gas pressureof said environment is at least a factor of five.
 5. A method as inclaim 3 wherein the pressure differential between said elevated pressureand said lesser gas pressure of said environment is a factor in therange of from 2 to 10¹⁰.
 6. A method as in claim 5 wherein the pressuredifferential is up to 10⁸ when the lesser gas pressure is subatmosphericor up to 10⁴ when the lesser gas pressure is substantially atmospheric.7. A method as in claim 1 wherein said steps a., b. and c. are repeated2-200 times.
 8. A method as in claim 7 wherein said steps a., b. and c.are repeated 10-100 times.
 9. A method as in claim 1 where each saidstep b. is continued for a period of 10 seconds to 10 minutes.
 10. Amethod as in claim 1 wherein said purging gas is a mixture of at leasttwo gases.
 11. A method as in claim 10 wherein one of said gases ispresent in said mixture in a concentration in the range of 5 ppm to 5percent of said mixture.
 12. A method as in claim 10, further comprisingthat the relative concentrations of the gases in said mixture does notsubstantially change during operation of said method.
 13. A method as inclaim 1 wherein said purging gas is a bulk gas, a speciality gas or agas mixture.
 14. A method as in claim 13 wherein said purging gascomprises hydrogen, oxygen, nitrogen, argon, hydrogen chloride, ammonia,air, carbon dioxide, helium, silane, germane, diborane, phosphine,arsine or mixtures thereof.
 15. A method as in claim 1 wherein saidsubsequent decontamination of said contaminated gas comprised reductionof concentration of contaminants to a level of no greater than 1 ppm.16. A method as in claim 15 wherein said subsequent decontamination ofsaid contaminated gas comprised reduction of concentration ofcontaminants to a level of on the order of 1-10 ppb.
 17. A method as inclaim 16 wherein said subsequent decontamination of said contaminatedgas comprised reduction of concentration of contaminants to a level ofon the order of 1-100 ppt.
 18. A method as in claim 1 wherein saidsubstrate is porous.
 19. A method as in claim 18 wherein said substratehas a surface area of at least 100 m²/g.
 20. A method as in claim 18wherein said method is used to accomplish activation of decontaminationsites on the surface of said substrate.
 21. A method as in claim 18wherein said method is used to accomplish preconditioning of saidsubstrate by purging its content of a packing gas.
 22. A method as inclaim 21 wherein preconditioning causes a chemical reaction whichgenerates a gaseous byproduct and accomplishing preconditioning of saidsubstrate comprises purging its content of said packing gas and of saidbyproduct.
 23. A method as in claim 1 wherein said steps a., b. and c.are repeated until the temperature within said vessel passes through amaximum value and decreases to a substantially constant equilibriumvalue.